Ultrasonic delay line having offset boundary surfaces at point where beam strikes surface

ABSTRACT

A delay member in which the path of travel of an ultrasonic wave beam from an input transducer to an output transducer associated with the member is lengthened by reflection of the beam from at least one of the planar boundary surfaces of the member. At least one of these surfaces is divided into at least two substantially parallel, but offset, surfaces so that a wave beam from the input transducer impinging on these offset surfaces is separated into at least two wave beams which travel over paths of different lengths to the output transducer. At least one of the separate beams is effective, at certain frequencies, to produce a counter voltage at the output transducer to attenuate, to the desired degree, the voltage developed at the output transducer and improve the transmission characteristic of the delay line.

Unied States Patent 1191 Probst 1 May 22, 1973 ULTRASONIC DELAY LINE HAVING 3,522,557 8/1970 Duncan et al ..333/30 R ()FFSET BOUNDARY SURFACES AT 3,071,741 1/1963 Brauer ..333/30 R 3,568,l04 3/1971 Bailey ..333/30 R SURFACE Primary Examiner-Paul L. Gensler I [75] Inventor: Helmut Probst, Ulm/Danube, A ttorney-Spencer &.Kaye

Germany 57 ABSTRA T [73] Ass1gnee: Licentla Patent-Verwaltungs Gmbl'l, 1 C

F kf Germany A delay member in which'the path of travel of an ultrasonic wave beam from an input transducer to an [22] filed 1971 output transducer associated with the member is 211 App], 175,203 lengthened by reflection of the beam from at least one of the planar boundary surfaces of the member. At least one of these surfaces is divided into at least two [30] Foreign Apphcmon Data substantially parallel, but offset, surfaces so that 21 Aug. 28, 1970 Germany ..P 20 42 865.3 wave beam from the input transducer impinging On these offset surfaces is separated into at least two 52 us. 01. ..333/30 R 333/71 Wave beam which travel Paths diffeem [51] Int CL; 6 9/30 lengths to the output transducer. At least one of the [58] Field 0 R 71 72 separate beams is effective, at certain frequencies, to produce a counter voltage at the output transducer to attenuate, to the desired degree, the voltage [56] References Clted developed at the output transducer and improve the UNITED STATES PATENTS transmission characteristic of the delay line.

2,839,731 6/1958 McSkimin ..333/30 R 4 Claims, 7 Drawing Figures Patented May 22, 1973 3,735,292

2 Sheets-Sheet 1 Patented May 22, 1973 OUTPUT VOLTAGE OUTPUT VOLTAGE 2 Sheets-Sheet 2 flMHz] FIG ULTRASONIC DELAY LINE HAVING OFFSET BOUNDARY SURFACES AT POINT WHERE BEAM STRIKES SURFACE BACKGROUND OF THE INVENTION The present invention relates to an ultrasonic delay line in which an ultrasonic beam transmitted through a delay member, from an electromechanical input transducer to an electromechanical output transducer, is reflected, at least once, at the planar boundary surfaces of the delay member. Ultrasonic delay lines in which the path of travel of ultrasonic wave beams can be extended by deflecting or reflecting the beams in a delay member are known. With such a system multiple reflection of the beam permits the spatial dimensions of the delay member to be smaller.

It is also known that the transmission characteristic of a delay line is generally similar to a bell-shaped curve, i.e. a signal of a limited bandwidth can be transmitted by such a delay line. Often however, this curve representing output voltage as plotted against frequency, has an asymmetric shape, caused by inaccuracies in the shape of the delay member or because of frequency dependent attenuation of the ultrasonic beam. When a delay line is used for a color television receiver, operating according to the PAL (phase alternating line) method, beams in the frequency range between approximately 3.4 and 5.2 MHz are transmitted. In this transmission frequency range it is desirable to have the curve of voltage as flat as possible, i.e. to make the frequency dependence as low as possible.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a novel ultrasonic delay line having an improved transmission characteristic.

According to the invention a delay member has a plurality of beam reflection surfaces at least one of which is divided into two, or more, substantially parallel, offset, partial reflection surfaces. With such an arrangement a beam of ultrasonic energy which is reflected from such surface is divided into a number of partial beams corresponding in number to the number of reflection surfaces. Because the reflecting surfaces are offset, the lengths of the paths of travel of the separate beams through the delay member will be different.

According to a preferred embodiment of the invention the difference in the length of the path between the two partial beams of ultrasonic energy and the ratio of the width of the two partial beam is so selected that the transmission characteristic in the passband is flattened.

When at least one reflection surface in a delay member is divided into steep reflection surfaces an ultrasonic wave beam impinging on the surfaces is divided. This produces parallel beams that travel through the delay member in paths of different lengths. With the appropriate selection of a difference in length of the paths of the beams and of the ratio of the beam widths a flattening of the transmission characteristic will be obtained in that the resulting interference will flatten the dome-shaped overly high portion of the transmission characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a front elevation view of a delay member, constituting a preferred embodiment of the invention.

FIG. 1a is a side elevation view of the delay member shown in FIG. 1.

FIG. 2 is an enlarged detail view of an offset portion of the reflection surface formed in the delay member shown in FIG. 1 and 2.

FIG. 3 is a diagram depicting the transmission characteristic of a delay member.

FIG. 3a is a diagram of the counter voltage produced by a member according to the invention, to shift the output voltage values in a direction to produce the desired characteristic of FIG. 3.

FIG. 4 is a view similar to FIG. 3 showing different transmission characteristic curves.

FIG. 4a is a view similar to FIG. 3a showing a different counter voltage which is produced to shift the output voltage values in the direction of the desired characteristic of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a delay member, of a material such as glass, providing an ultrasonic delay line resulting from a five-fold reflection of the ultrasonic beam. The ultrasonic energy is transmitted from an input piezoelectric transducer 2 to an output piezoelectric transducer 3 in the form of a beam, generally indicated at 4. The five reflection surfaces are a, b,-b c, d and e.

According to the present invention at least one of these reflection surfaces is divided into for example, two parallel offset partial reflection surfaces b and 12 as seen in FIGS. 1 and 2, in order to produce two resulting ultrasonic wave beams 5 and 6 that will have the different lengths of paths in the delay member 1.

FIG. 2 shows to an enlarged scale, the reflection surface (b, and b as divided into the two steep reflection surfaces b and b The two steep reflection surfaces b and b are parallel to one another and offset normal to their planes by a distance h. The impinging ultrasonic wave beam 4 is divided into two beam portions generally indicated at 5 and 6. The selection of the location and size of the step or offset between the two partial reflection surfaces b and [1 determines the ratio of the widths i and g of the two beam portions 5 and 6.

The relationships AI 2h cos a and AI An(c/f) An) V where AI the difference in the length of the path of travel for the ultrasonic beams 5 and 6;

An difference between the numbers of wavelengths of the ultrasonic wave beams 5 and 6;

h width of the space between the parallel offset reflection portions b and 12 f frequency of the ultrasonic waves whose wavelength is )t;

c speed of sound in member 1;

a impingement angle of the ultrasonic wave beam 4; produce the following equation for the height h:

h (An/2 cos a) c/f For those frequencies f at which the differences Al in path length is approximately An wavelength (A) where An l, 2, 3 the output voltage U at the delay line remains practically unchanged even when partial beams 5 and 6 are produced. However, frequencies exist therebetween for which AI (An i k). At these frequencies the output voltage U is weakened due to interference between beams 5 and 6.

By selecting the ratio between beam widths i' and g, the degree of correction can be adjusted and, predominantly by the selected difference in path length, the frequency range in which the transmission characteristic is flattened can be adjusted.

FIGS. 3 and 3a show a flattening of the curve depicting the transmission characteristic of the delay line according to the present invention in the case of a curve which initially has an overly high portion at the low frequency end of the transmission band. The original unflattened curve is shown in dashed lines and the flattened curve resulting from the modification according to the invention is shown in solid lines. FIG. 3a shows the countervoltage AU produced by ultrasonic wave beam 6 for the case where this beam has a path length which is 1.5K shorter than that of the ultrasonic wave beam at that frequency where the strongest attentuation of the output voltage is to occur. It can be seen that this frequency is slightly less than 4 MHz. In a preferred embodiment according to the invention a ratio of the widths i and g of beams 5 and 6 was chosen of about g/i A to 1/5.

FIGS. 4 and 4a shows the flattening of the transmission characteristic curve realized by the present invention, where the peak of the original curve occurs substantially in the center of the frequency transmission band. The unflattened transmission curve is again shown in dashed lines and the transmission curve which has been flattened according to the present invention is shown in solid lines. In this case the step height h is so selected that the difference in path length between the beams 5 and 6, for the center band frequency, is 2.5).. In the center of the transmission range the output voltage U is attenuated the most by the countervoltage AU while the output voltage remains practically unchanged at the band edges of 3.4 and 5.2 MHz, because the path lengths there differ by 2 and 3 whole wavelengths, respectively. In a preferred ultrasonic delay line for color television sets having a shape as shown in FIG. 1 in case of FIG. 3 and 3a a height h of about 0.69 mm and in case of FIG. 4 and 4a a height h of about 1.05 mm gave good results.

Generally, only a relatively low countervoltage AU is required to achieve a desired flattening of the transmission characteristic curve. Therefore the beam portion 6 which is split away from the ultrasonic beam 4 and which is required for the correction, is small compared to the main beam 5. The difference in path length which is produced by the step can be at the center of the frequency band, about 0.5 to 3.5 wavelengths.

It is also possible to effect an even more effective correction if, instead of utilizing only one step, two steps are applied to at least one of the reflection surfaces of the ultrasonic delay member.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Iclaim:

1. An ultrasonic delay line composed of a delay member having planar reflecting boundary surfaces and in which an ultrasonic wave beam, transmitted through the delay member between an electromechanical input transducer and an electro-mechanical output transducer, is reflected, at least once, at one of the planar boundary surfaces, the improvement wherein at least one of the planar boundary surfaces is divided into at least two substantially parallel, offset surfaces at a point where said surface is struck by the beam, the ultrasonic wave beam from the input transducer which impinges on said offset portions being reflected as at least two ultrasonic wave beams which travel over paths of different lengths from the input transducer to the output transducer, and said parallel offset surfaces are offset a preselected distance to provide wave beams of selected widths, the ratio of the widths and the differences in path lengths of the wave beams is such that at least one of the reflected ultrasonic wave beams is effective, within a defined frequency range, to produce electric transducers. 

1. An ultrasonic delay line composed of a delay member having planar reflecting boundary surfaces and in which an ultrasonic wave beam, transmitted through the delay member between an electromechanical input transducer and an electro-mechanical output transducer, is reflected, at least once, at one of the planar boundary surfaces, the improvement wherein at least one of the planar boundary surfaces is divided into at least two substantially parallel, offset surfaces at a point where said surface is struck by the beam, the ultrasonic wave beam from the input transducer which impinges on said offset portions being reflected as at least two ultrasonic wave beams which travel over paths of different lengtHs from the input transducer to the output transducer, and said parallel offset surfaces are offset a preselected distance to provide wave beams of selected widths, the ratio of the widths and the differences in path lengths of the wave beams is such that at least one of the reflected ultrasonic wave beams is effective, within a defined frequency range, to produce a voltage at the output transducer which is counter to the voltage produced by the remainder of the reflected beam, whereby the transmission characteristic of said line is flattened within such frequency range.
 2. An ultrasonic delay line as defined in claim 1 wherein the delay member has five planar outline surfaces for reflecting ultrasonic wave beam energy.
 3. An ultrasonic delay line as defined in claim 1 wherein the delay member consists of glass.
 4. An ultrasonic delay line as defined in claim 1 wherein the electromechanical transducers are piezoelectric transducers. 